Matthew Moocarme

Ultralow-Intensity Magneto-Optical and Mechanical Effects in Metal Nanocolloids

Magneto-plasmonics is a designation generally associated with ferromagnetic-plasmonic materials because such optical responses from nonmagnetic materials alone are considered weak. Here, we show that there exists a switching transition between linear and nonlinear magneto-optical behaviors in noble-metal nanocolloids that is observable at ultralow illumination intensities and direct current magnetic fields. The response is attributed to polarization-dependent nonzero-time-averaged plasmonic loops, vortex power flows, and nanoparticle magnetization. This work identifies significant mechanical effects that subsequently exist via magnetic-dipole interactions.

Plasmon-induced Lorentz forces of nanowire chiral hybrid modes

The mechanical forces associated with surface currents are widely overlooked and point to a new family of plasmonically-driven processes. Here, we investigate the Lorentz forces acting on a free electron gas that is bound to the surface of a nanowire. We demonstrate that appreciable mechanical forces are produced by longer illumination wavelengths between longitudinal and transverse absorption resonances via the excitation of chiral hybrid plasmon modes. We are the first to associate plasmonic activity as the underlying mechanism for nanowire rotation, which explains prior experimental results. The presence of chiral hybrid plasmon modes yields the greatest net translation and torque forces. The asymmetric plasmon behavior subsequently affects the complex nonlinear dynamics of plasmonic nonspherical nanoparticles in fluids.

Anomalously-large photo-induced magnetic response of metallic nanocolloids in aqueous solution using a solar simulator.

We experimentally, analytically, and numerically demonstrate the nonlinear photo-induced plasmon-assisted magnetic response that occurs with metallic nanoparticles in aqueous solution. We measure the scattered spectra from solutions of gold nanospheres (10(-7) fill factor) and observe appreciable changes when simultaneously applying DC magnetic fields and illuminating samples with light. The magnetic response is achieved using light from a solar simulator at unprecedented low illumination intensities (< 1W/cm(2)) and is sustained when the magnetic field is removed. Distinctly different behavior is observed depending on the circular-polarization handedness given a fixed magnetic field. Nanoparticle aggregation is more likely to occur when the circular-polarization trajectory opposes the solenoid current that produces the magnetic field. Using Mies theoretical solution, we show how vortex orbital surface currents lead to an increased and anisotropic electrical conductivity, which shifts the scattered spectra in agreement with experimental results. The single-nanoparticle plasmon-induced magnetization, which couples the scattered and incident electric fields, changes sign with orthogonal circular-polarization handedness.

Control of photo-induced voltages in plasmonic crystals via spin-orbit interactions

There is wide interest in understanding and leveraging the nonlinear plasmon-induced potentials of nanostructured materials. We investigate the electrical response produced by spin-polarized light across a large-area bottom-up assembled 2D plasmonic crystal. Numerical approximations of the Lorentz forces provide quantitative agreement with our experimentally-measured DC voltages. We show that the underlying mechanism of the spin-polarized voltages is a gradient force that arises from asymmetric, time-averaged hotspots, whose locations shift with the chirality of light. Finally, we formalize the role of spin-orbit interactions in the shifted intensity patterns and significantly advance our understanding of the physical phenomena, often related to the spin Hall effect of light.

Meta-Optical Chirality and Emergent Eigen-polarization Modes via Plasmon Interactions

The response of an individual meta-atom is often generalized to explain the collective response of a metasurface in a manner that neglects the interactions between meta-atoms. Here, we study a metasurface composed of tilted achiral meta-atoms with no spatial variation of the unit cell that derives appreciable optical chirality solely from the asymmetric interactions between meta-atoms. The interactions between meta-atoms are considered to stem from the Lorentz force arising from the Larmor radiation of adjacent plasmonic resonators because their inclusion in a simple model accurately predicts the bonding/anti- bonding modes that are measured experimentally. We also experimentally observe the emergence of multiple polarization eigenmodes, among other polarization-dependent responses, which cannot be modeled with the conventional formalism of transmission matrices. Our results are vital to the precise characterization and design of metasurfaces.

Influence of solvent polarity on light-induced thermal cycles in plasmonic nanofluids

Pattern formation often reveals constituent nonlinear mechanisms of a complex system. Here, we study self-synchronizing, light-induced thermal cycles in plasmonically absorbing nanofluids, whose anomalous thermal, optomechanical, electrochemical, and hydrodynamic responses are not yet well understood. We show that the oscillatory behavior—caused by light grazing the nanofluid meniscus—exhibits a strong dependence on hydrogen bonding in the solvent environment and that there are low-intensity optical thresholds in alcohol–water binary-solvent nanofluids. Moreover, these thermal cycles occur with a periodic, vertically discharging heat-dissipation mechanism, which could be facilitated by nanobubbles or thermophoresis. We show that an incoherent white-light source, such as sunlight, will also induce self-synchronizing thermal cycles; in this demonstration, we illustrate new methods of energy storage, transfer, and harvesting that will not alter the natural carbon cycle of life.

Spontaneous light-driven heat cycles in metallic nanofluids with nanobubbles

We present the first experiments of spontaneous oscillatory behavior in binary-solvent nanofluids, which occurs when collimated light grazes menisci. The robust heat cycles identify nanobubbles, new mechanisms for probing nanoparticle-solvent chemistry, and novel thermo-mechanical dynamics.

Robustness and spatial multiplexing via diffractal architectures

When plane waves diffract through fractal-patterned apertures, the resulting far-field profiles or diffractals also exhibit iterated, self-similar features. Here we show that this specific architecture enables robust signal transmission and spatial multiplexing: arbitrary parts of a diffractal contain sufficient information to recreate the entire original sparse signal.

Anomalously-large photo-induced magnetic response of disperse metallic nanocolloids

We demonstrate for the first time a plasmon-assisted magnetic response that occurs with disperse gold nanoparticles in aqueous solution. We observe increased Fano-like resonances and show the nonlinear interaction that occurs with matrix vortices.